- #1
Abid Rizvi
- 20
- 0
Homework Statement
The "spirit-in-glass thermometer", invented in Florence, Italy, around 1654, consists of a tube of liquid (the spirit) containing a number of submerged glass spheres with slightly different masses (see the figure below). At sufficiently low temperatures all the spheres float, but as the temperature rises, the spheres sink one after the other. The device is a crude but interesting tool for measuring temperature. Suppose that the tube is filled with ethyl alcohol, whose density is 0.78945 g/cm3 at 20.0° and decreases to 0.78097 g/cm3 at 30.0°C.
If one of the spheres has a radius of 1.400 cm and is in equilibrium halfway up the tube at 20.0°C, determine its mass?
When the temperature increases to 30.0°C, what mass must a second sphere of the same radius have in order to be in equilibrium at the halfway point?
At 30.0°C the first sphere has fallen to the bottom of the tube. What upward force does the bottom of the tube exert on this sphere
Homework Equations
buoyant force = mg
The Attempt at a Solution
So I go the first 2 correct.
For the first one I had: M_sphere*g = p_alcohol_at_20 * g * 4/3 pi r^3
solving for M_sphere, I got 9.07 grams
For the second one I had M_sphere2*g = p_alcohol_at_30 * g * 4/3 pi r^3
solving for M_sphere2, I got 8.98 grams
For the last part, I set up this equation:
buoyant force + normal force = mg_sphere
p_alcohol_at_30*V_sphere*g + N = M_sphere*g
But this equation has 2 unknowns, N (the normal force) and p_sphere, the spheres density. I'm not even a 100% sure if 1.4cm is the radius of this sphere so I may not even know the volume meaning 3 unknowns... What am I missing?
Thanks in advance